A large amount of boron compounds are used in each of electroplating, enameled ironware, glaze, clay roof tile, and electrical component manufacturers, for example. Wastewater generated in the manufacturing process contains a high concentration of boric acid, borax, and the like. In accordance with the revision of the Water Pollution Prevention Law, an effluent standard value of boron, which has not been a subject to be regulated thus far, has been set to 10 ppm. Thus, there is a need for a technology that reduces the concentration of boron in boron-containing wastewater to a low concentration.
The methods of removing boron from boron-containing wastewater are broadly classified into a flocculation/precipitation method (batch method) using a flocculation/precipitation agent and an adsorption method (column method) using an adsorption resin, in the same manner as in methods of removing other heavy metals from water. For the former method, aluminum sulfate, which is inexpensive, is used. For the latter method, there are used a resin and a fiber each having an N-methyl glucamine group, for example.
It is an advantage of the flocculation/precipitation method that a large amount of boron-containing wastewater can be easily treated. Boron in wastewater can be removed by: adding a flocculation agent, a flocculation aid, a pH adjustor, and the like to an appropriate flocculation/precipitation vessel loaded with boron-containing wastewater, to thereby flocculate boron; and separating boron and water from each other by spontaneous precipitation or centrifugation. However, as the concentration of boron in wastewater becomes lower owing to the removal of boron, the flocculation to be formed decreases. Therefore, in order to reduce the concentration of boron to the effluent standard value or less, a large excess amount of flocculation agents must be added, which simultaneously generates a large amount of sludge and increases treatment cost. This has been a problem. Further, the flocculation rate is rate-limiting, and hence, a long retention time is required in a flocculation/precipitation vessel. This has been another problem.
On the other hand, it is an advantage of the adsorption method that the method involves, as an adsorption group, for example, N-methyl glucamine capable of specifically binding to a borate ion, and hence can reduce the concentration of boron to an extremely low concentration. However, N-methyl glucamine is expensive, and hence, an adsorbent using N-methyl glucamine causes an increase in treatment cost. In addition, in the adsorption method, the treatment of a large amount and a high concentration of boron-containing wastewater may saturate an adsorption group in a short time.
Therefore, in practice, a part of business sectors or companies that can afford to invest in treatment equipment have introduced a combination method involving: removing a majority of boron by such a flocculation/precipitation method as mentioned above; and then removing a low concentration of boron by the adsorption method.
In the above-mentioned background, there is a strong demand for the development of a boron adsorbent that exhibits an excellent boron adsorbing ability even in a low concentration of boron-containing water, is inexpensive, and has a high versatility, and a method of removing boron capable of simply and efficiently removing boron.